This invention relates to probes of the type which are adapted to be driven by an AC signal and extend into the materials whose condition is being measured. More specifically, this invention is directed to an apparatus adapted to be driven by a radio frequency signal so as to measure changes in capacitance or susceptance as a function of the condition of materials within a vessel.
Probes of the RF type are disclosed in U.S. Pat. Nos. 4,064,753 and 4,166,388 assigned to the assignee of this invention. The probes as disclosed therein are essentially flexible and include longitudinally extending probe and guard electrodes which are surrounded with insulation. These probes are best suited for use in conductive materials which provide a radio frequency ground in intimate contact with the measuring region of the probe insulation. Therefore, measurement of insulating materials do not conveniently lend themselves to the use of such probes. For example, some vessels have non-vertical walls which cause an undesirable nonlinearity of capacitance change per change in material level when using a probe which measures to the walls of the vessel. It may also be difficult to avoid movement of the probe with respect to the wall and this too can create inaccuracies in measuring the condition of materials. Furthermore, many vessels are not grounded, necessitating the installation of a ground or reference electrodes in a uniform geometrical relationship. For vessels which are suitable for using such a probe, it is not possible to reliably calibrate the probe until the probe is installed within the vessel since the exact geometrical relationship between the probe and the vessel is not known.
Another probe for measuring the levels of liquids within a vessel is disclosed in U.S. Pat. No. 3,918,306 which is assigned to the assignee of this invention. The probe includes exposed parallel probe and ground electrodes for measuring the level of fluid between the exposed electrodes. Such probes may not be used with conductive liquids. Moreover, the probe structure including the ground and probe electrodes is substantially rigid so as to make it difficult to insert an elongated probe vertically into a vessel having a minimal overhead clearance. Moreover, elongated probes (e.g., probes having a length of 20 feet or more) are not readily manufacturable and transportable. Furthermore, such probes which incorporate a guard electrode which is not coextensive with the ground and probe electrodes are adversely affected by coatings which collect on the probe above the level of the material whose condition is being measured. See also, U.S. Pat. No. 3,285,068 and U.S. Pat. No. 3,079,797 for a disclosure of rigid probe and ground electrodes where at least one of the electrodes is exposed to the fluid whose level is being measured.
U.S. Pat. No. 4,208,909 is also assigned to the assignee of this invention and discloses the use of a level measuring probe as well as a dielectric constant compensating probe. However, the probe does not include a guard electrode interposed between any probe electrode and ground electrode. Moreover, the probe is rigid posing more of the above-discussed problems.